Brown is expected to go to Buckingham Palace to ask the Queen to dissolve Parliament following a cabinet meeting later today. He will then confirm the widely-predicted date.
Fleishman-Hillard head of public affairs Nick Williams, who is a former senior political adviser to Tony Blair's Labour Party, said: ‘Traditionally elections are not simply won or lost on the election campaign alone, but over a period of time. However, given the current narrow opinion polls what happens in this campaign could be the difference between a working parliamentary majority and a hung Parliament meaning this campaign is the most important and hardest fought for 13 years.'
Williams added that Brown's visit to the Queen will be followed by a well-publicised visit or speech to an electorally important constituency.
‘Each of the opposition leaders will do exactly the same,' said Williams. ‘These visits have been carefully orchestrated by each team for months.'
Weber Shandwick, chairman UK corporate comms and public affairs, Jon McLeod added: ‘Today's starting gun is, of course, a mere formality as we were already underway as of 1 January. However, the key task for the parties is to reach undecided swing voters, as the polls at the moment reflect only those who are 100 per cent certain to vote.'
‘For David Cameron, that means getting them to press to button marked 'change'. For Brown it means persuading them to stick with the devil-you-know. Nick Clegg's task is more complex, as he has a series of crucial local battles to win, while persuading the nation that the Lib Dems are fit to participate in some form of power-sharing.'
Opinion polls have varied considerably in recent days, suggesting that either the Conservatives or Labour could lead a hung parliament or that the Tories may win a small majority.
Insight Public Affairs account director Oliver Kendall added: 'Nick Clegg's goal will be to get the Big Mo. Broadcast election rules should ensure parity of coverage with the other two and already interest in the LibDems has been piqued because of the prospect of a hung parliament.
'Although no politician will ever admit to paying too much attention to polls, look out for the YouGov daily tracker poll. If the first two weeks of the campaign show growing LibDem support, then print journalists will be forced to ditch their out-of-date two-party mentality and start talking seriously about the LibDems.'